Protective roles of CX3CR1-mediated signals in toxin A-induced enteritis through the induction of heme oxygenase-1 expression

The injection of Clostridium difficile toxin A into the ileal loops caused fluid accumulation with the destruction of intestinal epithelial structure and the recruitment of neutrophils and macrophages. Concomitantly, intraileal gene expression of CX3CL1/fractalkine (FKN) and its receptor, CX3CR1, was enhanced. When treated with toxin A in a similar manner, CX3CR1-deficient (CX3CR1(-/-)) mice exhibited exaggerated fluid accumulation, histopathological alterations, and neutrophil recruitment, but not macrophage infiltration. Mice reconstituted with CX3CR1(-/-) mouse-derived bone marrow cells exhibited exacerbated toxin A-induced enteritis, indicating that the lack of the CX3CR1 gene for hematopoietic cells aggravated toxin A-induced enteritis. A heme oxygenase-1 (HO-1) inhibitor, tin-protoporphyrin-IX, markedly increased fluid accumulation in toxin A-treated wild-type mice, indicating the protective roles of HO-1 in this situation. HO-1 expression was detected mainly in F4/80-positive cells expressing CX3CR1, and CX3CR1(-/-) mice failed to increase HO-1 expression after toxin A treatment. Moreover, CX3CL1/FKN induced HO-1 gene expression by isolated lamina propria-derived macrophages or a mouse macrophage cell line, RAW264.7, through the activation of the ERK signal pathway. Thus, CX3CL1/FKN could induce CX3CR1-expressing macrophages to express HO-1, thereby ameliorating toxin A-induced enteritis.